Taehyun Jeon

Channel estimation for MIMO-OFDM systems employing spatial multiplexing

Multiple-input multiple-output (MIMO) communication methods based on orthogonal frequency division multiplexing (OFDM) and spatial multiplexing (SM) can lead to a substantial improvement in the throughput performance of many existing wireless channels. The main strength of MIMO systems based on SM is their ability to support very high data rates. Accurate channel estimation is essential in realizing the full performance potential of MIMO-OFDM. Channel estimation also becomes a major challenge as the number of parallel wireless links increases substantially with the number of antennas at the transmitter and receiver. In this paper we develop a channel estimation algorithm well suited for MIMO-OFDM based on the same preamble and signal field structure as the IEEE 802.11a standard, the current single-input single-output wireless local area network (WLAN) standard. This constraint is motivated by the backward compatibility requirement of the upcoming IEEE 802.11n standard, the next generation WLAN standard that is currently being developed. In addition to the training symbol and the signal field symbol, we make use of the soft symbol information available in the data portion of the packet in updating the channel estimate in a sequential and decision directed fashion. It is shown that when this approach is tested against a 4/spl times/4 (4 transmit antennas and 4 receive antennas) SM configuration, the channel responses of all sixteen parallel wireless links can he estimated with sufficient accuracy, starting with only one OFDM training symbol.

Design and prototype development of MIMO-OFDM for next generation wireless LAN

Multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) is regarded as a promising technology enabling higher data rate wireless communications in frequency selective fading channels. This paper proposes the next generation wireless local area network (WLAN) physical (PHY) layer transmission technology using dual-band and MIMO-OFDM schemes and discusses test results of MIMO-OFDM prototype system implemented with field programmable gate arrays (FPGA). This design targets over 200 Mbps of the maximum PHY data rate in 40 MHz bandwidth and compatibility with the legacy IEEE 802.11a. For a cost-effective implementation and improved packet error rate (PER) performance, 2 transmit and 3 receive antennas and 40 MHz channel are used to achieve the desired PER and throughput performance in the 5 GHz band.

Sequence detection for binary ISI channels using signal-space partitioning

Binary symbol detection based on a sequence of finite observation signals is formulated in the multidimensional signal space. A systematic space partitioning method is proposed to divide the entire space into two decision regions using a set of hyperplanes. The resulting detector structure consists of K parallel linear classifiers followed by a K-to-1 Boolean mapper, and is well suited to high-speed implementation. Compared to direct implementation of the fixed-delay tree search (FDTS) detection rule, the proposed signal-space formulation results in a considerable saving in digital hardware. Examples taken from binary-input intersymbol interference (ISI) channels are used to demonstrate the proposed technique. Block processing strategies suitable for high-speed applications are also discussed.

A Systematic Approach To Signal Space Detection

A general procedure is developed to implement the fixed delay tree search (FDTS) detector in the multi-dimensional signal space. The procedure systematically finds a required set of hyperplanes that divide the entire space into different decision regions. A Boolean mapping function is also analytically determined which generates a final binary decision from several intermediate binary variables that indicate the position of the observation vector relative to given hyperplanes. The proposed technique is demonstrated on a (1+D)/sup 2/ partial response channel driven by the (1,k)-coded input.

Carrier phase and frequency recovery for MIMO-OFDM

A new carrier phase and frequency recovery scheme is proposed for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The algorithm uses an extended Kalman filter (EKF) to estimate the instantaneous phase and frequency offset. A linear minimum mean squared error (LMMSE) data-channel extractor is used to isolate the phase and frequency dependent term from the received signal. The proposed algorithm can provide both phase and carrier frequency recovery and has lower complexity than other approaches in the literature. In addition there is very little latency associated with the proposed approach. Simulation results show that the proposed algorithm can track quickly carrier phase and frequency offset and the performance loss compared to the synchronous case is very small.

Transmit antenna selection for MIMO systems with V-BLAST type detection

In an attempt to improve the spectral efficiency of wireless communication systems, MIMO spatial multiplexing architectures have been introduced. V-BLAST is proposed as an efficient receiver structure in terms of performance and computational complexity. We introduce a transmit antenna selection scheme for a V-BLAST type receiver with an assumption that channel information is available at the transmit side. With a given MIMO channel matrix, we derive a simplified channel capacity and vector symbol error probability in V-BLAST. Using these results, the paper provides the optimal criteria for transmit antenna selection in terms of capacity and error probability. These criteria are based on tap weight vectors in V-BLAST detection. Additionally, we propose sub-optimal methods that achieve near-optimal performance with low computational load. To verify the efficiency of the proposed schemes, computer simulation results are presented.

High accuracy and low complexity timing offset estimation for MIMO-OFDM receivers

This paper presents a high accuracy and low complexity timing offset estimation algorithm for multi input multi output orthogonal frequency division multiplexing (MIMO-OFDM) receivers in the tracking mode. First of all we propose an effective architecture of phase tracking for MIMO-OFDM receivers. Then we describe two timing offset estimation algorithms; pilot-aided and CFO-aided timing offset estimation. The first algorithm is a timing phase estimation that can minimize the effect of distorted scattered pilots due to severe fading. We show that the performance of the proposed pilot-aided timing offset estimation very close to that of an offset-free system. The second algorithm attempts to use the computation result of carrier frequency offset estimation in order to reduce complexity. The proposed CFO-aided timing offset estimation has very low complexity comparable to that of the pilot subcarrier aided schemes without performance degradation

BER performance comparisons in digital tape recording

Bit-error-rate (BER) performances of various DC free coding and detection methods have been compared in digital tape recording environments. Performance sensitivities to additive noise, head-tape space variations and offtrack interference have been investigated at two representative linear densities. The results indicate that the performance ranking of different techniques depends strongly on the type of interference assumed in the channel. In particular, the fixed delay tree search with decision feedback (FDTS/DF) detector used in conjunction with the rate 8/10 code performs comparably to or better than the partial response maximum likelihood technique combined with the rate 24/25 code in all cases considered, despite the significantly lower code rate.

An Low Complexity Hardware Implementation of MIMO Detector with Application to WLAN

In this paper, we describe a FPGA implementation of MIMO detector for future wireless communication system with application to wireless LAN, targeted for upcoming 802.11n standard. The MIMO detector assumes 2 transmit and 3 receive antennas. In soft-output demapper, we apply channel state information which effectively weights reliability information to soft-decision output bits for enhanced link-level performance. The implementation complexity is significantly reduced by avoiding repeated pseudo-inverse calculation for interference cancellation of every received symbol vector. Furthermore, the overall processing time and fabrication area it takes can be significantly reduced by applying bit reduction technique